A 1.0108 g sample of an unknown nonelectrolyte is dissolved in 10.0090 g of benzophenone and produces a solution that freezes at 35.8 C. If the pure benzophenone melted at 48.1 C, what is the molecular weight of the unknown compound?

100.98 g/mol

Explanation:

Solution:

1) Determine temperature change:

   48.1 - 35.8 = 12.3 °C

2) Determine how many moles of the compound dissolved:

   Δt = i Kf m

   1.674 °C = (1) (9.8 °C kg mol¯1) (x / 0.010009 kg)

   9.8 °C = (979.12 °C mol¯1) (x)

   x = 0.010009 mol

3) Determine molecular weight:

   1.0108 g / 0.010009 mol = 100.98 g/mol

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